1. Field of the Invention
This invention relates generally to frequency and code acquisition in a wireless portable communication device. More particularly, the invention relates to multiple simultaneous frequency and code acquisition in a code division multiple access (CDMA) communication system.
2. Related Art
With the increasing availability of efficient, low cost electronic modules, mobile communication systems are becoming more and more widespread. For example, there are many variations of communication schemes in which various frequencies, transmission schemes, modulation techniques and communication protocols are used to provide two-way voice and data communications in a handheld, telephone-like communication handset. The different modulation and transmission schemes each have advantages and disadvantages.
Regardless of the type of communication system, a portable transceiver operating in a communication system must acquire a signal from a transmitter located at a base-station, so that the portable transceiver may synchronize, with respect to frequency and time (or code for a CDMA communication system), with the communication network. It is desirable to minimize the amount of time required for the portable transceiver to synchronize to the network. One of the main factors that influences the amount of time needed for synchronization is the accuracy of the frequency reference source in the portable transceiver. The frequency reference source is typically referred to as an “oscillator.” If the frequency reference source is highly accurate, fewer frequencies must be searched to acquire the signal from the base station. For example, some transceivers use a temperature controlled crystal oscillator (TCXO) to derive the proper frequency reference signal. Generally, a TCXO is accurate to within a few parts per million (ppm), such that frequency acquisition time is minimized. For example, a frequency reference source that is accurate to within 2-3 ppm allows less frequency drift than a frequency reference source that is accurate to within 20 ppm.
Unfortunately, the accuracy of the oscillator is directly proportional to its cost. In other words, a highly accurate TCXO will cost significantly more than, for example, a capacitor controlled crystal oscillator (CCXO), which may have a frequency accurate to within, for example, 15-20 ppm.
With the continued drive in the industry toward reducing cost, it would be desirable to reduce the cost of the portable communication device by reducing the cost of the oscillator. For example, in a portable communication device, it would be desirable to use a CCXO instead of a TCXO as the frequency reference source. Unfortunately, because the CCXO is less accurate than other available frequency references sources, in some circumstances the frequency acquisition time of a portable communication device using a CCXO may exceed the allowable time.
In a 3G application for a system operating in the wideband code division multiple access (WCDMA) communication system, a synchronization channel, or signal, referred to as the synchronization channel (SCH), is used for cell search and code acquisition. The SCH comprises two sub-channels, the primary SCH (PSCH) and the secondary SCH (SSCH). In WCDMA, a 10 millisecond (ms) radio frame for the PSCH and the SSCH is divided into 15 slots, each slot having a length of 2560 chips. A WCDMA radio frame is shown in Table 1.
The PSCH comprises a modulated code having a length of 256 chips and is referred to as the primary synchronization code (PSC), denoted as cp, and is transmitted once per slot. The PSC is the same for every cell in the WCDMA system.
TABLE 1
The SSCH comprises the repeated transmission of a sequence of codes having a length of 256 chips and is referred to as the secondary synchronization code (SSC). The SSC is referred to as csi,k where i=0, 1, . . . 63 is the number of the scrambling code group, and k=0, 1, . . . 14 is the slot number. The SSC is transmitted in parallel with the primary synchronization code (PSC) in the PSCH. Each SSC is chosen from a group of 16 different codes having a length of 256 chips. The SSC sequence on the SSCH indicates to which of the code groups the cell's downlink scrambling code belongs.
The primary and secondary synchronization codes are modulated by a symbol referred to “a.” The symbol “a” indicates the presence or absence of space time transmit diversity (STTD) encoding on the primary common control physical channel (P-CCPCH) and is given by the following Table 2.
TABLE 2P-CCPCH STTD encodeda = +1P-CCPCH not STTD encodeda = −1
Regarding channel acquisition, including frequency and primary code (PSC) acquisition on the PSCH, if a large frequency error exists between a transmitter and receiver, the correlation of the 256 chips will suffer significant degradation. Because the PSC only transmits one symbol (256 chips in this example) in each slot, it typically must be averaged over many slots (for example, 60 slots) to be accurately decoded. In such a case, the frequency error causes the symbols in the received signal to drift. Furthermore, averaging the PSC over many slots causes significant performance degradation.
Therefore, it would be desirable to minimize the acquisition time of a portable communication device, while allowing the use of a low-cost frequency reference source.